Potassium nitrate (KNO3) is a commercially important chemical with uses from explosives to fertilizers. Polyhalite (K2SO4.MgSO4.2CaSO4.2H2O) is a widely-distributed and readily available source of potassium. Reaction of polyhalite with acids (e.g. HNO3) is known in the literature as a means of producing crude salt mixtures containing “complex mineral fertilizer.” For example, U.S. Pat. No. 4,246,019 discloses a method for production of a mixture containing 53.54% KNO3, 39.87% Mg(NO3)2, 5.48% CaSO4, and 1.01% H2O from the reaction of polyhalite with HNO3. Russian Pat. No. 2,276,123 discloses a method for production of a solution containing a mixture of K2SO4, MgSO4 and NH4NO3 from reaction of polyhalite with HNO3 followed by neutralizaion with ammonia. Thus the processes described in the literature allow to produce a solution containing a mixture of substances, including potassium and magnesium nitrates, as well as gypsum that had not been separated in early stages of the process.
Several well-known processes, such as the Southwest Potash and IMI processes, are known for the production of KNO3 by reaction of KCl with HNO3 (Ullman's Agrochemicals, vol. 1; Weinheim: Wiley-VCH, 2007, pp. 334-336). Production of essentially pure KNO3 directly from the reaction of polyhalite with strong acid remains unknown, however. Thus, there remains a long-felt need for a process that can produce essentially pure KNO3 from polyhalite without the complications of known processes such as production of complicated product mixtures and necessity of neutralization with ammonia.